In an air drier of the type with two drying towers containing desiccant one of the towers is used for drying air emanating from the compressor, while at the same time the desiccant in the other tower is regenerated by a small fraction of the dried air, which is passed back through the desiccant. The switch-over between the two towers is to be accomplished by a valve, which may also have the function to vent out the regenerating air from the tower that for the time being is not used for drying the air.
A switch-over valve may--as is well known in the art--be controlled either entirely by the air pressure in the system (in which case no separate and external control means are required) or by external control means, which may be electrical and/or pneumatic. The latter control type is to be used when it is desired to accomplish a switch-over at other instances than at a certain air pressure level in the system.
In order to safe-guard a proper function and to minimize the forces acting on the different components during the switch-over process, it is preferred to perform the switch-over when the air drier is unloaded by means of the unloader valve; the switch-over is in other words correlated with the unloading.